BACKGROUND: There are few studies on the long-term associations of physical activity (PA) to cognition. Here, we examine the association of midlife PA to late-life cognitive function and dementia. METHODS: The sample consisted of a population-based cohort of men and women (born in 1907-1935) participating in the Age Gene/Environment Susceptibility-Reykjavik Study. The interval between the midlife ascertainment of PA and late-life cognitive function was 26 years. Composite scores of speed of processing, memory, and executive function were assessed with a battery of neuropsychological tests, and dementia was diagnosed according to international guidelines. There were 4,761 nondemented participants and 184 (3.7%) with a diagnosis of dementia, with complete data for the analysis. RESULTS: Among the participants, no midlife PA was reported by 68.8%, 5 hours PA by 4.5%. Excluding participants with dementia compared with the no PA group, both PA groups had significantly faster speed of processing (5 hours, beta = .32, p trend